Synthesis of oligo(phenyleneethynylene)-tetrathiafulvalene cruciforms for molecular electronics

Article abstract of DOI:10.1021/ol060071o

Novel oligo(phenyleneethynylene) (OPE)-tetrathiafulvalene (TTF) cruciform molecules containing thiol end-groups have been prepared and characterized. These redox-active molecules are interesting for future applications as molecular wires/transistors for m

Full text of DOI:10.1021/ol060071o

ORGANIC
LETTERS
2006
Vol. 8, No. 6
1173-1176
Synthesis of
Oligo(phenyleneethynylene)-Tetrathiafulvalene
Cruciforms for Molecular Electronics
Jakob Kryger Sørensen, Mikkel Vestergaard, Anders Kadziola, Kristine Kilså, and
Mogens Brøndsted Nielsen*
Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5,
DK-2100 Copenhagen Ø, Denmark
mbn@kiku.dk
Received January 11, 2006
ABSTRACT
Novel oligo(phenyleneethynylene) (OPE)-tetrathiafulvalene (TTF) cruciform molecules containing thiol end-groups have been prepared and
characterized. These redox-active molecules are interesting for future applications as molecular wires/transistors for molecular electronics.
The rational design and synthesis of molecular wires,
rectifiers, switches, and transistors for molecular electronics
has attracted immense interest in recent years.1 Much work
has focused on thiol-terminated conjugated π-systems, such
as oligo(phenylenevinylene)s (OPVs),2 oligo(phenylene-
ethynylene)s (OPEs),3 and oligothiophenes.4 The electronic
conduction through a molecular wire may be controlled by
attachment of suitable functional groups along its backbone.
Thus, Reed, Tour, and co-workers5 have demonstrated
conductance on-off switching in devices based on wires
functionalized with NH2 and NO2 groups.6
Nuckolls and co-workers7 recently introduced a new class
of molecules consisting of two perpendicularly disposed
π-systems, bis-phenyloxazole and terphenyl, for molecular
(3) (a) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.;
Monnell, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara,
D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303-2307. (b)
Chanteau, S. H.; Tour, J. M. J. Org. Chem. 2003, 68, 8750-8766. (c) Stuhr-
Hansen, N.; Sørensen, J. K.; Moth-Poulsen, K.; Christensen, J. B.;
Bjørnholm, T.; Nielsen, M. B. Tetrahedron 2005, 61, 12288-12295. (d)
Wang, C.; Batsanov, A. S.; Bryce, M. R. J. Org. Chem. 2006, 71, 108-
116.
(1) (a) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804. (b) Carroll, R.
L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378-4400. (c)
Robertson, N.; McGowan, C. A. Chem. Soc. ReV. 2003, 32, 96-103. (d)
Flood, A. H.; Stoddart, J. F.; Steuerman, D. W.; Heath, J. R. Science 2004,
306, 2055-2056. (e) Nørgaard, K.; Bjørnholm, T. Chem. Commun. 2005,
1812-1823. (f) Elbing, M.; Ochs, R.; Koentopp, M.; Fischer, M.; von
Ha¨nisch, C.; Weigend, F.; Evers, F.; Weber, H. B.; Mayor, M. Proc. Nat.
Acad. Sci. U.S.A. 2005, 102, 8815-8820.
(4) (a) Purcell, S. T.; Garcia, N.; Binh, V. T.; Jones, L., II; Tour, J. M.
J. Am. Chem. Soc. 1994, 116, 11985-11989. (b) Hicks, R. G.; Nodwell,
M. B. J. Am. Chem. Soc. 2000, 122, 6746-6753.
(2) (a) Stuhr-Hansen, N.; Christensen, J. B.; Harrit, N.; Bjørnholm, T.
J. Org. Chem. 2003, 68, 1275-1282. (b) Kubatkin, S.; Danilov, A.; Hjort,
M.; Cornil, J.; Bre´das, J.-L.; Stuhr-Hansen, N.; Hedegård, P.; Bjørnholm,
T. Nature 2003, 425, 698-701.
(5) (a) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999,
286, 1550-1552. (b) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y.;
Jagessar, R. C.; Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C.-W.; Chen,
J.; Wang, W.; Campbell, I. Chem. Eur. J. 2001, 7, 5118-5134.
10.1021/ol060071o CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/18/2006

wide applications in both materials and supramolecular
chemistry.6a,11
The cruciform target molecule 1 contains an OPE3
functionalized with two dithiafulvene donor groups at the
lateral positions and acetyl-protected thiol end-groups. The
thiol group is commonly employed as the anchoring group
for adhesion onto gold electrodes. The single molecule
conductivity is expected to depend on the redox-state of the
orthogonal extended TTF moiety (0, +1, +2). Simply put,
two-electron oxidation is expected to change the π-electron
delocalization along the OPE backbone from linearly con-
jugated to cross-conjugated (i.e., quinodal structure in the
central ring).
The synthesis of the wire 1 proceeds according to Scheme
1. First, the triflate 212 was subjected to a palladium-catalyzed
Sonogashira cross-coupling13 with trimethylsilylacetylene to
provide compound 3.14 The ester groups were then reduced
with LiAlH4 (or alternatively with (i-Bu)2AlH (DIBAL-H)),
which gave the diol 4 in almost quantitative yield. Oxidation
with pyridinium chlorochromate (PCC) furnished the di-
aldehyde 5 that was subsequently treated with the readily
available phosphonium salt 615 and diethylamine to afford
the extended TTF 7 in a 2-fold Wittig reaction. Desilylation
upon treatment with K2CO3 in MeOH/THF gave the product
8 precipitating from the reaction mixture. This compound
exhibits a remarkable stability and can be stored at room
temperature for several weeks without apparent decomposi-
Figure 1. Target molecule 1 derived from OPE3 and TTF.
electronics applications. Other examples of such cruciform
molecules were prepared by Bunz and co-workers,8 who
reported a whole class of OPE3s with orthogonally oriented
arylvinylenes. We became interested in exploiting the redox-
active tetrathiafulvene (TTF) donor together with an OPE3
wire in new cruciforms for molecular electronics (Figure 1),
which is a continuation of our work on acetylenic scaffolds
of benzene-extended TTFs.9,10 TTF exhibits two reversible
one-electron oxidations steps and has for this reason found
Scheme 1. Synthesis of OPE3-TTF Cruciform
1174
Org. Lett., Vol. 8, No. 6, 2006

tion in sharp contrast to previously reported TTFs containing
terminal alkyne functionalities.9,16 Compound 8 was then
subjected to a 2-fold palladium-catalyzed cross-coupling with
the bromide 9,2a employing slightly modified reaction
conditions of Hundertmark et al.,17 which provided the
OPE3-TTF cruciform 10. We have recently employed similar
conditions for synthesizing the parent OPE3 (11, Figure 2)
subjecting 10 to these conditions gave the compound 1. The
Ac groups of 1 can be removed upon treatment with HNEt2/
CHCl3 (1:1).
Single crystals of 7 were subjected to an X-ray crystal-
lographic analysis. The structure (Figure 3) reveals planarity
of the complete π-system.
Figure 2. Thiol-terminated OPE3.
with t-Bu-protected thiol end-groups.3c Addition of NaOMe
(1 equiv) increased the yield of 10 from 26% to 56%.
Moreover, we applied ultrasonification as we have previously
demonstrated its ability to promote the Sonogashira cross-
coupling.16b
One advantage of using t-Bu as protecting group is the
resistance of t-Bu-S-Ar to both strongly basic and acidic
conditions.2a Furthermore, the OPE3-TTF 10 presents a target
molecule in itself for fundamental conductivity studies, as
Kubatkin et al.2b have demonstrated that t-BuS-functionalized
wires can be physisorbed onto electrodes by a weak van der
Waals contact between the single molecule and the device.
However, in a final synthetic step, the t-BuS group can be
converted into the AcS moiety by means of AcCl/BBr3. Thus,
Figure 3. X-ray crystal structure of 7 (CCDC 294502). Drawing
made by ORTEP-II.18
UV-vis absorption spectra show that the presence of
donor groups in 10 results in a significant decrease in the
HOMO-LUMO gap relative to that of 11. The lowest energy
absorpton maxima occur at 411 nm (very broad, 3.0 eV)
and 320 nm (fine-structured, 3.9 eV) for 10 and 11,
respectively.
Cyclic voltammetry of 10 in CH2Cl2 reveals a single
irreversible oxidation (scan rate 0.1 V s-1). According to
differential pulse voltammetry, the oxidation occurs at 0.47
V vs Fc+/Fc and is likely a two-electron process. No
oxidation peaks are observed for 11 under similar conditions.
The irreversible oxidation observed in bulk solution for 10
should not be directly linked to its applicability in single-
molecule electronics where the molecule is isolated between
gold electrodes.
A computational study was performed to elucidate the
frontier orbitals of the OPE3-TTF cruciforms. Compounds
10 and 11 were geometry-optimized at the semiempirical
PM3 calculational level using the Gaussian program pack-
age.19 Frontier orbitals (Figure 4) were obtained by density
functional theory (DFT) single-point calculations at the
B3LYP/6-31+g(d,p) level. Whereas the HOMO is situated
along the wire for 11, this orbital has the strongest coef-
ficients at atoms vertically to the wire in 10, i.e., at the
extended TTF donor moiety. This observation substantiates
the rationale behind the molecular design. In contrast, the
LUMO of 10 resides mainly along the wire and resembles
to a large extent that of 11. Single-point energy calculations
on 10, 10•+, and 102+ (all in the neutral conformation)
provide first and second vertical ionization energies (IE) of
(6) For other examples of conductance switching by oxidation/reduction,
see: (a) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C.
P.; Heath, J. R. Acc. Chem. Res. 2001, 34, 433-444. (b) McCreery, R.;
Dieringer, J.; Solak, A. O.; Snyder, B.; Nowak, A. M.; McGovern, W. R.;
DuVall, S. J. Am. Chem. Soc. 2003, 125, 10748-10758. (c) Dinglasan, J.
A. M.; Bailey, M.; Park, J. B.; Dhirani, A.-A. J. Am. Chem. Soc. 2004,
126, 6491-6497. (e) Reference 2b.
(7) Klare, J. E.; Tulevski, G. S.; Sugo, K.; de Picciotto, A.; White, K.
A.; Nuckolls, C. J. Am. Chem. Soc. 2003, 125, 6030-6031.
(8) (a) Wilson, J. N.; Josowicz, M.; Wang, Y.; Bunz, U. H. F. Chem.
Commun. 2003, 2962-2963. (b) Wilson, J. N.; Bunz, U. H. F. J. Am. Chem.
Soc. 2005, 127, 4124-4125.
(9) (a) Nielsen, M. B. Synlett 2003, 1423-1426. (b) Nielsen, M. B.;
Petersen, J. C.; Thorup, N.; Jessing, M.; Andersson, A. S.; Jepsen, A. S.;
Gisselbrecht, J.-P.; Boudon, C.; Gross, M. J. Mater. Chem. 2005, 15, 2599-
2605.
(10) For synthesis and characterization of the parent benzene-extended
TTF, see: Salle, M.; Belyasmine, A.; Gorgues, A.; Jubault, M.; Soyer, N.
Tetrahedron Lett. 1991, 32, 2897-2900.
(11) (a) Bryce, M. R. AdV. Mater. 1999, 11, 11-23. (b) Nielsen, M. B.;
Lomholt, C.; Becher, J. Chem. Soc. ReV. 2000, 29, 153-164. (c) Bryce,
M. R. J. Mater. Chem. 2000, 10, 589-598. (d) Segura, J. L.; Mart´ın, N.
Angew. Chem., Int. Ed. 2001, 40, 1372-1409.
(12) Zhang, Q.; Shi, C.; Zhang, H.-R.; Wang, K. K. J. Org. Chem. 2000,
65, 7977-7983.
(13) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
16, 4467-4470.
(14) For a crystal structure of the i-Pr3Si derivative, see: Nicolaisen,
B.; Bond, A. D.; Nielsen, M. B. Acta Crystallogr. 2004, E60, o784-o785.
(15) Sato, M.; Gonnella, N. C.; Cava, M. P. J. Org. Chem. 1979, 44,
930-934.
(16) (a) Nielsen. M. B.; Gisselbrecht, J.-P.; Thorup, N.; Piotto, S. P.;
Boudon, C.; Gross, M. Tetrahedron Lett. 2003, 44, 6721-6723. (b)
Andersson, A. S.; Qvortrup, K.; Torbensen, E. R.; Mayer, J.-P.; Gisselbrecht,
J.-P.; Boudon, C.; Gross, M.; Kadziola, A.; Kilså, K.; Nielsen, M. B. Eur.
J. Org. Chem. 2005, 3660-3671.
(17) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org.
Lett. 2000, 2, 1729-1731.
(18) ORTEP-II. A Fortran Thermal-Ellipsoid Plot Program. Report
ORNL-5138. Johnson, C. K. Oak Ridge National Laboratory, Oak Ridge,
TN 1976.
Org. Lett., Vol. 8, No. 6, 2006
1175

In conclusion, stable OPE3-TTF cruciform wires with thiol
end-caps have been prepared from a sequence of high-
yielding steps. Exploitation of both 1 and 10 as redox-
controlled switches for molecular electronics is currently
under investigation.
Acknowledgment. The Danish Research Agency is
acknowledged for financial support (Nos. 2111-04-0018 and
21-04-0084).
Supporting Information Available: Experimental pro-
cedures, NMR and UV-vis spectra, and X-ray crystal data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL060071O
(19) Gaussian 03, Revision B.03: Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.,
Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;
Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.
J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian,
Inc., Pittsburgh, PA, 2003.
Figure 4. Calculated HOMO and LUMO orbitals (B3LYP/6-
31+g(d,p)) for the compounds 10 (left) and 11 (right).
6.07 and 8.60 eV, respectively. The vertical IEs of the parent
TTF (devoid of the CO2Me substituents) were previously
calculated to be 6.49 and 11.10 eV, respectively.20 Hence,
compound 10 is a better electron donor (gas phase) than TTF
despite the electron-withdrawing CO2Me groups. For com-
parison, a vertical IE of 6.46 eV is obtained for 11.
(20) Nielsen, S. B.; Nielsen, M. B.; Jensen, H. J. Aa. Phys. Chem. Chem.
Phys. 2003, 5, 1376-1380.
1176
Org. Lett., Vol. 8, No. 6, 2006